The present invention relates to noise suppression systems. In particular, the present invention relates to noise suppression systems for use with gas turbines such as aircraft auxiliary power units (APUs).
Large commercial aircraft typically include on-board APUs, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the APU provides the main power source for a variety of systems and serves as a main engine starter. The APU may also provide power during in-flight operations for various systems.
For commercial passenger aircraft in particular, there is widespread demand by the airline industry to maintain noise levels generated by turbines below defined limits. This is particularly important at ground service stations for the aircraft, where ground crews load and unload luggage, fuel and provision the aircraft. When the aircraft is on the ground, minimizing APU noise is therefore desirable.
One technique for attenuating the exhaust noise of an APU involves placing an exhaust silencer directly downstream from the APU exhaust diffuser. This allows the exhaust silencer to attenuate the noise of the combustion gases as the gases exit the exhaust diffusers. The exhaust silencer is typically placed directly downstream from the APU exhaust diffuser to minimize the overall axial length of the APU. However, the combustion gases exit the APU at high velocities, and create a turbulent mixing downstream from the exhaust diffuser. The turbulent mixing generates a substantial amount of additional noise downstream from the exhaust diffuser.
To address this issue, it is common to increase the size of the exhaust silencer, and thereby increase the size of the APU housing structure so as to suppress noise generated downstream of the exhaust diffuser. However, such a solution undesirably increases the overall size and weight of the aircraft tail section.